Touchpad is well known and widely used in various electronic products. Touchpad provides a simple, easy and cheap pointing device, such as those in laptop computer systems for mouse function. FIG. 5 shows a schematic diagram of the construction of a touchpad and the operational principle thereof, in which an insulator plate (not shown) is arranged on the top of the functional stack consisting of a ground plate 36, a layer of traces 38 along Y-axis, a layer of traces 40 along X-axis, all of them connected to a control circuit 46 apart from the panel 12. An equivalent capacitance 42 presents between the Y-axis trace 38 and the ground plate 36, and an equivalent capacitance 44 presents between the X-axis trace 40 and the ground plate 36. Once finger or conductor touching the panel 12, the control circuit 46 determines the touched position of the finger or conductor by sensing the change in capacitances of each corresponding traces 38 and 40 and responds for further process.
There are three types of touchpads, i.e., resistive, electromagnetic and capacitive. The capacitive touchpad was applied for touching mode monitors such as those in public Internet phones and guiding systems. However, the capacitive touchpad lacks of handwriting function. The resistive touchpad has been used for touching mode monitors such as the applications in personal digital assistants (PDA) and electronic dictionaries. Unfortunately, the resistive touchpad cannot be inputted by light touching of users'fingers. Currently, the modules available for handwriting recognitions and keypads on tablet PCs and electronic books are all implemented with resistive or electromagnetic touchpads. The resistive touchpad is operated only by focused-point pressing on its panel, and the electromagnetic touchpad is operated by a battered input pen.
In consideration of functionality and cost, the capacitive touchpad is obviously preferred over the resistive and electromagnetic touchpads. The operational principle of the capacitive touchpad employs the instant electric change produced by capacitive effect in response to a user's finger or conductor's touch onto the panel of the touchpad to detect the position the finger or conductor is touching. Unlike the electromagnetic touchpad, the capacitive touchpad is easy to interface for users and needs not a power-consumed input pen. On the other hand, interfacing onto the capacitive touchpad requires no focused pressed point as that to the resistive touchpad and thus gives the capacitive touchpad a longer lifetime. Moreover, the capacitive touchpad has simple construction, fewer components, and higher production yield, and as a result, lower cost is achieved for mass production thereby.
By the aforementioned illustration, obviously, the capacitive touchpad has more advantages than the other two. If the capacitive touchpad is modified to be integrated with more input functions, such as keyboard and handwriting, a low-cost and easy-to-use input interface could be provided.